The present invention relates to polymer latex coating compositions and, more particularly, to latices of vinylidene chloride polymers.
In recent years, vinylidene chloride polymer latices have been widely used, particularly in the food industry, as coatings for plastic films, cellophane, paper, and like packaging materials due to their high resistance to chemicals and oils, low temperature heat-sealing properties, and excellent resistance to the transmission of oxygen and moisture vapor. However, it has also been recognized that such coatings tend to be generally deficient in a number of properties demanded by advanced packaging techniques. For instance, such coatings should have a lower dynamic coefficient of friction to permit faster packaging speeds. They should also be capable of forming low temperature heat-seals having higher strength to withstand the stresses inherent in such packaging speeds. Further, they should have reduced blocking tendencies to permit rapid feeding from aged rolls of coated packaging material without breaking or destroying the coatings.
Nevertheless, it is generally considered that heat-sealing properties are incompatible with antiblocking and slip properties. Accordingly, the methods chosen to modify such coatings typically have focused on improving only a limited number of properties at one time to the detriment of other properties, thereby inherently restricting the resulting coatings to specific end-uses.
For example, one method consists of increasing the vinylidene chloride content of the latex polymer to improve crystallinity and reduce the tendency for blocking. Due to such increased crystallinity, however, this method undesirably reduces the heat-sealing properties.
Another method employs a suitable amount of an additive, such as talc, wax, silica, or the like, to reduce blocking and increase slip properties. Such additives are typically incompatible with the latices and, accordingly, have a tendency to affect latex stability and reduce the transparency of the resulting coatings.
A further method has consisted of blending a latex having reduced blocking tendencies but poor heat-sealing properties with one or more latices having good heat-sealing properties but high blocking tendencies to achieve a suitable balance in overall performance properties. Although this method has proven to be generally successful in its results, it is nevertheless disadvantageous inasmuch as it necessitates preparing several different but compatible latices and requires careful control of the proportions of each during blending.
Accordingly, it is an object of the present invention to provide a vinylidene chloride polymer latex which can be used to form coatings having improved slip properties and reduced blocking tendencies without sacrificing the low temperature heat-sealing properties.